I downloaded the latest version ImageOnMap-4.1.jar and i have this error in console when admin joins to server.

Plugin also sends me a broken message:

My server is running on Paper 1.16.5

Already fixed in zDevelopers/QuartzLib#74; we'll release a new 4.1.1 version today.
The plugin will still work as expected and only server administrators see this message.

Thanks anyway as you took time to report this!